In digital transmission, particularly as applied to telephony, recommended standards have been developed for the transmission of pulse code modulated (PCM) signals. A standard signal bit stream having a level designation DS-1 is formed from a plurality of 24 words or channels having a level designation of DS-0. In each of the transmission systems, the frame repetition rate =8000 Hz (frame period=125 us) and the number of PCM bits per channel n=8. The following are additional parameters of some of the standard transmission bit streams.
______________________________________ DESIG- BIT RATE CHANNELS OVERHEAD BITS NATION (kb/s) PER FRAME PER 125 us ______________________________________ DS-0 64 1 -- DS-1 1544 24 1 DS-1C 3152 48 10 DS-2 6312 96 21 DS-3 44736 672 216 ______________________________________
As shown, each of the multi-channel bit streams at the various bit rates include some additional overhead bits which provide synchronization and signalling control. In one higher order system two tributaries at the DS-1 rate may be multiplexed together with some additional control signals to form a bit stream at the DS-1C bit rate. Similarly, four tributaries at the DS-1 rate may be multiplexed to form a bit stream at the DS-2 rate. Likewise, seven DS-2 tributaries may be multiplexed together with further control bits to form a bit stream at the DS-3 bit rate. Thus the existing standard hierarchy is comprised of a plurality of signal levels, each one of which is generated by multiplexing a specified number of the next lowest signal level in the hierarchy in such a manner that all signals below that level are no longer accessible in the multiplexed structure without completely demultiplexing the signal. Many of the additional control bits are utilized to signal the insertion of additional bits (i.e. pulse stuffing) in the bit streams of each of the tributaries in order to increase their bit rates to a common higher bit rate so that the signals from the tributaries may be directly interleaved even though they may be asynchronous with each other or the higher-order bit stream.
The techniques for both pulse stuffing and multiplexing are well known and have been described in U.S. Pat. No. 3,136,861 entitled "PCM Network Synchronization" issued Jun. 9, 1964 to John S. Mayo. A typical prior art system utilizing these techniques is shown in FIG. 1, which illustrates an exemplary portion of a multiplexed digital transmission system in which a plurality of tributaries at the primary bit rate are multiplexed to second and third order bit rates. In the system, PCM signals from twenty-four 64 kb/s channels CH1, CH2-CH24, are multiplexed in a multiplexer M1 together with a synchronization bit to form a DS-1 bit stream at its output. Up to four such tributaries are bit stuffed in a stuffer S1 and then multiplexed in a multiplexer M2 together with further control bits to produce a DS-2 bit stream at its output. Seven such tributaries are then further bit stuffed in a stuffer S3 and multiplexed in a multiplexer M3 together with still further control bits to produce a DS-3 bit stream. The DS-3 bit stream is then transmitted from transmitter Tx to receiver Rx whereupon it is demultiplexed back down to the individual channel levels through destuffers T2 and T1, and demultiplexers D3, D2, and D1, in a converse manner. Similar multiplexing structures may be used to generate even higher-order bit streams utilizing the same principles.
This prior art arrangement functioned well in the past because it spread the stuffing bits throughout the bit stream thereby minimizing the requirements for memory buffers which were a significant cost factor in earlier systems. However such systems lacked a great deal of flexibility since it was not possible to recover selected channels directly from the higher-order bit stream without demultiplexing each of the intermediate order bit streams of the tributaries containing those channels. The problem was exacerbated if the selected channels were from different tributaries. Such prior art multiplex schemes also had to sacrifice networking features and characteristics for bandwidth efficiency due to the constraints of transmission equipment whereas, with modern systems bandwidth efficiency can be traded for networking features such as ease of implementation of the cross-connect and ease of access to digital switches.
In certain applications, there is often a requirement to access only a few such channels at an intermediate point along a system. In prior systems it was necessary to demultiplex and destuff the signals from each higher-order level before the next lower-order level could be demultiplexed. It was not possible to go directly from the received bit stream to the channel level in a single demultiplexer. As a result, there was considerable overhead required to access even one channel of the higher-order bit stream.
In a converse arrangement, both bit stuffing and multiplexing at each of the intermediate levels, were required in order to insert even a few channels from one or more tributaries in the higher-order bit stream. Such an arrangement severely limited the flexibility of any integrated digital transmission and digital switching system where it was desired to either switch or cross-connect individual channels or tributaries without demultiplexing the entire bit stream through each of the individual stages.